1. The Field of the Invention
The present invention relates generally to radio modulators. More specifically the present invention is related to methods and apparatus for dynamically calibrating an In-phase and Quadrature (I/Q) modulator to suppress the carrier frequency component before the transmission of information across a communication channel.
2. The Relevant Technology
Radio communication systems are well known within the prior art and are invaluably useful to modern society as a means of conveying information from one location to another. Integral, in some form, with every radio communication system is a radio modulator at the transmission end for converting information into radio transmission form and, at the receiving end, a radio demodulator for converting the radio transmission form back into information. While modulators and demodulators convert information into radio transmission form and back again, most present-day radio communication systems additionally include circuitry, known as frequency mixers, to translate information frequencies into other frequencies and back again. Mixers exist in various embodiments and, depending upon the specific embodiment, may or may not allow the frequencies of the input signals to appear at the output. Double balanced mixers are one such exemplary mixer embodiment and are used to illustrate the problem herein. As is well known, a double balanced mixer separates input signals into two different channels, the in-phase and quadrature channels, but as described herein, will only be referenced as having a singular channel.
With reference to FIG. 1, a frequency spectrum output of an exemplary doubly balanced mixer is depicted generally as 20 with sidebands 22 being approximated by an envelope of a curve 24 that assumes a generally sin (x.sup.2 /x) shape. With proper modulating techniques the carrier component 26 at frequency, fc, is suppressed. The problem, however, is that in many hardware devices incorporating a mixer means for suppressing the carrier component have failed or badly deteriorated. With reference to FIG. 2, it can be seen that an improperly suppressed carrier component 28 at the carrier frequency, fc, yields a corresponding power magnitude having an amplitude level A, commonly known as the DC offset. Although not problematic for all types of radio communication systems, amplitude level A is often unacceptably high when compared to the power magnitude of the information sidebands which are generally around amplitude level B. For example, in some present day devices, such as radio modems, the DC offset at amplitude level A is as much as 15-20 dB, or more, above the power magnitude of the information at amplitude level B. This power amplitude difference is frequently attributed to internal components that are subject to internal variations every time the device is powered on. Such components include, but are not limited to, self-calibrating digital-to-analog convertors (DACs) which are typically arranged in electrical precedence to the modulator. Since no information is contained in the carrier component and since the power magnitude at the carrier frequency is numerous decibels above information curve envelope 30, efficiency in the transmitter is lost when devices attempt to accommodate the excessive power magnitude of the "information-less" carrier component.
Radio transmitters typically also have final stage amplifiers electrically subsequent to the modulator or mixer stage to boost the power levels of the information before transmitting the information in radio form across a communication channel. Thus, an economic and componentry burden is introduced with amplifier circuitry that accommodates an overly high DC offset.
Moreover, since the carrier frequency is centrally positioned within the frequency range containing the information, the carrier component is not frequency filtered in the transmitter by any band pass filters. Yet the excessive power magnitude is still accommodated by componentry within these filters. For these and other similar reasons, it is desirable to suppress the carrier component.
The prior art has long been aware of these and other problems and has thus attempted various means to eliminate them. Suppression of the carrier component before transmission, however, still remains problematic. One known suppression means employs circuitry-related calibration coefficients that are pre-determined at the time of design for suppressing the DC offset at the carrier frequency. Although usually initially effective, the magnitude of the carrier is subject to drift over time and the calibration coefficients are eventually rendered ineffective. Moreover, since the calibration coefficients are pre-determined they are usually preset and installed at the manufacturing level. Thus, a user is unable to adjust the calibration coefficients and must seek factory repair or recalibration at an additional expense.
Temperature compensation circuits are also employed to suppress the carrier component, but they too are pre-determined and established at the time of installation and are similarly subject to drift characteristics over time. Thus, repair or recalibration comes with the foregoing described added expense.
Sometimes the carrier suppression is accomplished by an eclectic arrangement of both temperature compensation circuits and calibration coefficients and other related parameters such as the voltage supply. These arrangements, however, are complex in design and implementation thereof likewise leads to added expense.
On many older and some present day radio communication devices, an adjustable device, usually a potentiometer, is installed to allow the user to manually adjust and compensate for transmission related problems. But this too is subject to drift characteristics because the potentiometer is mechanically tuned and subject to being "un-tuned" by mechanical vibrations exerted upon the device. Such vibrations are exerted regularly during events such as jostling, bumping and carrying. Although the user may readjust or "tweak" the tuning setting, this takes time and is subject to human inaccuracies. The tweaking also requires that the user know when the device actually requires an adjustment which frequently remains unknown, especially concerning fine tuning adjustments. Manufacturing costs are also a concern with user calibration devices because increased costs typically accompany user calibration components. Such increased costs typically include the extra design for those components and the extra labor to install them.
Accordingly, it would be an advance to have a cost effective means of suppressing the carrier component before information is transmitted across a communication channel. It would also be an advance to provide for accurate calibration of the modulator, by other than factory installed or repair means, to allow calibration to be economically performed after the purchase of the device whenever it is required.